Sounds like you live in a high altitude: 85 octane fuel is not normally sold in areas where the altitude is less than 4000 feet.
The answer to your question depends on the altitude where you drive. If it is more than 5,000 or 6,000 feet; 87 octane would not be expected to give better mileage than 85. But if it is 4,000 feet or lower; 87 might do a little better than 85. If you mostly drive at altitudes between 4,000 and 5,000 feet; it would be difficult to say; you'd have to try a few tankfuls and compare the results.
Sometimes the brand of fuel makes more difference than the octane. Shell would potentially give the best mileage; but that sometimes varies in different brands of cars. Some off brand stations sell old fuel; and that could give worse mileage than major brand fuel of the same octane.
If you can find a station which sells gasoline that does not have ethanol added; you can expect an increase in mileage. You can find a list of stations that sell gas which does not contain ethanol at http://www.pure-gas.org/